The role of interferon and viruses in immunoregulatory disorders is being studied. Interferon has been found in the circulation of patients with certain autoimmune diseases and defective interferon production occurs in vitro in certain disorders. Retinitis pigmentosa is a disease of unkwown etiology. Peripheral leukocytes from patients with this disease when stimulated with mitogens produce subnormal amounts of interferon-gamma. These findings suggest that altered immune reactivity may be one of the abnormalities associated with this disease. Leukocytes from patients with acquired immunodeficiency syndrome (AIDS) also produce subnormal amounts of interferon-gamma in response to T-cell mitogens. This defect in the lymphokine cascade is frequently associated with persistent cytomegalovirus infection of leukocytes. An understanding of this defect may be important in modulating the impaired T lymphocyte-monocyte interaction in this disease.